^84 SECONDARY CHANGES. 



perennial woody stems. But not all the species of these genera are Lianes, nor have all 

 which are so an anomalous structure ; many have only an angular or lobed, but other.-, 

 wise normal xylem. 



Of the 145 species of the genus Serjania 84 have the above-described compound xylem, 

 five have divided xylem. The structure of the xylem, and especially the modifications 

 of the compound xylem, in number, relative size, special form and structure of the outer 

 rings, are always correlated with the other characters, according to which the subsidiary 

 groups of the genus are separated. The five species with divided xylems also form a 

 natural group according to other characters. 



Of the still more numerous species of Paullinia twelve have an anomalous and com- 

 pound xylem. 



Thinouia is characterised by the above-described subsequent appearance of cortical 

 zones of wood at the periphery of the compound wood ; whether this applies to all the 

 8-10 species of the genus is not certain. 



Of the species of the genus Urvillea, which are also 8-10 in number, anomalous struc- 

 ture is known only in U. laevis, and here it differs fundamentally from that of the other 

 Sapindaceae, and will be noticed in Sect. 193. 



Sect. 189. In the stem of the Calycanthem, as described on p. 257, the leaf-trace 

 bundles form the bundle-ring, while four of those bundles traverse the cortex. The 

 former developes into a normal wood and bast with normal cambium ; the cortical 

 bundles grow by partial cambiums into those cortical bundles discovered by Mirbel, 

 and since his time so frequently described^; these grow in thickness as long as the 

 stem lives. The cortical bundle is collateral, and is composed at first of a small 

 xylem and a stronger phloem, extended transversely; its orientation is inverted, 

 i. e. the phloem is turned inwards, towards the ring of wood, and the xylem outwards. 

 The latter directly abuts externally upon the broad, almost even, inner face of a strong 

 bundle of sclerenchymatous fibres, which is almost half-moon shaped in transverse 

 section : it is so surrounded by the phloem that the lateral margins of the latter also 

 are in contact with the fibrous bundle. The limiting layer between xylem and 

 phloem remains active as cambium, forming wood on the side next the former, and 

 on the side of the phloem a zone of bast, which embraces the wood. The cortical 

 bundle grows in this way like a single normal strand of wood surrounded by its 

 corresponding strand of bast, but retains the inverted orientation. Further, it retains 

 approximately, in its transverse section, the form of a broad and blunt triangle, which 

 it has from the first, if the fibrous bundle be included, and leaving out of account 

 irrelevant changes and irregularities, which appear as the volume increases. In the 

 first, and onwards to about the fifth year, the xylem of the bundle is extremely small 

 compared with the phloem, consisting of only few elements, while the section of the 

 latter already shows radial rows of many elements. Later it increases greatly in 

 strength ; it assumes the structure of a normal secondary strand of wood, divided by 

 small medullary rays, and even shows annual rings. The bast-zone surrounding 

 it only consists of elements of soft bast, and in the main at least of parenchymatous 

 elements : sieve-tubes remain still to be sought for. Its older layers suffer changes 

 of dilatation as in the normal bast, according as they pass away from the zone of 

 cambium ; by their own growth, and by the pressure caused by the layer of periderm 



' Compare p. 2J7. A- complete list of the literature is given by Woronin, /. c. 



